The present invention relates to a novel heterojunction device using .pi.-conjugated system polymer material as semiconductor materials.
It is thought that .pi.-conjugated system polymers comprise conjugated double bonds in their chemical structural skeleton and have a band structure consisting of a valence band formed by overlapping bonding .pi.-electron orbitals, conduction bands formed from antibonding .pi.-electron orbitals and a band gap which separates the two bands. Since the band gap is in the range of 1.5-4 eV depending on the material, .pi.-conjugated system polymers themselves are insulators. However, it is believed that carriers which transport charges are produced either by extracting electrons from the valence band (oxidation) or by injecting electrons into the conduction band (reduction) by means of chemical methods, electrochemical methods or the like (hereinafter referred to as doping). As a result, the conductivity of said .pi.-conjugated system polymers can be varied at will, over a mid range from the insulator region regime to the metal region regime by controlling the quantity of doping. When doping is performed by an oxidizing reaction, p-type polymers are produced, while reducing reactions produce n-type polymers. This relationship is similar to the addition of an impurity in an inorganic semiconductor. Because of this fact, it is possible to produce a semiconductor device using a .pi.-conjugated system polymer as the semiconductor material.
For example, a Schottky type junction device using polyacetylene (J. Appl. Phys. vol. 52, p. 869, 1984; Japanese patent application Laid-Open No. 56-147486, etc.) and a Schottky type junction device using a polypyrrole conjugated polymer (Japanese patent application Laid-Open No. 59-63760) have been heretofore known. A hetero-junction device prepared by combining n-CdS as an inorganic semiconductor with p-type polyacetylene has also been reported (J. Appl. Phys. vol. 51, P. 4252, 1980). A p-n homo junction device using p-type and n-type polyacetylene prepared by combining .pi.-conjugated system polymers themselves together has also been known (J. Appl. Phys. Lett. vol. 33, P. 18, 1978). This junction device however, is unstable even under the atmosphere of dried helium, leading to the loss of junction properties within a few hours.
In the above described conventional semiconductor device using .pi.-conjugated system polymers, most junctions are conbinations of .pi.-conjugated system polymers and a metal or of .pi.-conjugated system polymers and an inorganic semiconductor. The former is the Schottky type junction device which, although easy to prepare, is unstable during operation. For example, in the Schottky type junction formed between a p-type polypyrroleconjugated system polymer and indium, the junction properties thereof deteriorate even in a vacuum due to the reaction of the p-type dopant with indium (J. Appl. Phys., 56, p. 1036 (1984)).
As mentioned above, in a p-n homo junction device using p-type and n-type polyacetylene prepared by combining .pi.-conjugated system polymers themselves, the junction properties thereof deteriorate even in an inert atmosphere. This is because the mutual diffusion of p-type dopant anions and n-type dopant cations occurs at the interface of a p-n homo junction device which combines p-doped polyacetylene with n-doped polyacetylene (Appl. Phys. Lett., 33, p. 18 (1978)). Therefore, it was believed that a stable junction device prepared by combining .pi.-conjugated system polymers could not be formed because of the mutual diffusion of dopant ions occurring through the interface of the polymers by depositing two kinds of .pi.-conjugated system polymers. Moreover, it was believed that a stable junction device, in which the junction properties thereof were controlled on the basis of the amount of doping to .pi.-conjugated system polymers could not be formed.